1. Scope of the Invention
The invention is in the field of integrated optical components. More particularly, it relates to a wavelength division multiplexer and demultiplexer for combining or separating optical signals with different wavelengths in an optical waveguide.
2. Prior Art
Wavelength division multiplexers and demultiplexers for optical signals are known per se and are at present already commercially available in integrated form. They are used to increase the signal transmission capacity of an optical link, and they are intended, for example, for combining or separating signals from the two infrared windows, i.e. 1300 and 1550 nm. As disclosed in section S.2 of reference [1] ( see below under C), they often work with directional couplers or Mach-Zehnder interferometers. The selective suppression of the unwanted wavelength is, however, often insufficient. The output channels are therefore fitted with grooves in which a grating filter is placed. This does, however, have the disadvantage that an integrated optical component is combined with a bulk element. Also known are components in which such gratings are designed in integrated form with the help of submicron technology. These have, however, a complicated structure, and their fabrication is correspondingly laborious.
Reference [2] discloses a wavelength-demultiplexer based on a bimodal optical fibre. This demultiplexer combines a wavelength-selective mode conversion achieved in a resonant bimodal optical fibre, with mode splitting provided by means of a bimodal fused directional coupler. As such, this known fibre-optical demultiplexer cannot be used in the above-mentioned application, as there is no bimodal fibre for the communication wavelengths in the said infrared windows. It is true that an integrated optical version of a wavelength-demultiplexer based on this known principle can be obtained by using, for the purpose of the desired wavelength-selective mode conversion, a readily integratable mode converter of a type such as that described in a reference [3] not published in time. A bimodal directional coupler can also, in principle, be integrated. Since, however, the coupling behaviour desired is very specific, and is based on interference, an integrated optical form does require extremely small fabrication tolerances. Moreover, a demultiplexer based on this known principle cannot, if the direction of propagation of the light is reversed, be used directly as a multiplexer. Furthermore, because of the presence of the directional coupler, no polarisation-independent multiplexer and demultiplexer can be provided on the basis of the known principle.